In the production of copper, copper-bearing ore is mined, concentrated, smelted and refined. The copper-bearing ore contains many other elements, including many other metals, that must be separated from the copper before the copper is ready for sale. Each step of the copper producing process involves separating these other elements from the copper until the copper reaches a desired sale purity, typically four “nines” purity, i.e., 99.99% copper.
The smelting stage of the copper producing process produces a copper anode which is typically three hundred plus pounds and 98+ percent copper. These anodes contain, among other elements, various metals some of which are quite valuable. In order to separate these other elements from the copper, these anodes are placed into a tank (or “electrolytic cell”) of electrolyte containing sulfuric acid and subjected to a direct current (DC). Under the influence of the electrolyte and electric current, the copper anode dissolves and the copper plates onto a stainless steel cathode to form a copper cathode of four nines purity. The other elements in the copper anode precipitate to the bottom of the tank and form “anode slimes”.
Among the components of the anode slimes are such valuable metals as gold and silver and over the years, much effort has been directed to their recovery. These efforts have included both pyrometallurgical and hydrometallurgical methods each of which has its own advantages and disadvantages.
One hydrometallurgical process of interest is that developed by Kennecott Utah Copper and first described by J. E. Hoffmann et al. at the 1995 International Conference of The Metallurgical Society of CIM, Hydrometallurgical Processing of Kennecott Refinery Slimes, COPPER 95-COBRE 95 (Vol. III). In this process a slurry of the slimes and water (or an aqueous solution of hydrochloric acid) is formed, and the slurry is vigorously agitated with chlorine or hydrogen peroxide (a procedure known as “wet chlorination”). The chlorinated (or liberated) gold is extracted from the resulting mixture in a multi-stage, countercurrent flow scheme in which dibutyl carbitol (DBC) is the extracting organic solvent. The gold-loaded DBC is then scrubbed with an aqueous acid wash to remove minor metal impurities, the gold reduced directly from the DBC, and recovered.
Variations on this theme have been developed over the years, e.g., U.S. Pat. No. 5,942,024, but the search for improvements continues. For example, a reduction in the footprint of the physical plant in which the process is conducted is desirable both from a capital and operating cost perspective and from the amount of gold held in “inventory”. The larger the physical plant, the costlier to build, operate and maintain the plant, and the more gold is held within the plant.
Another source for improvement is in the recovery of gold from the slimes. In the current process, some gold-loaded DBC is entrained in the aqueous phase of the extraction process, and this can result in a loss of 5 to 20 ppm gold in the raffinate. Any recovery of this “lost” gold can result in a marked improvement to the overall economics of the process.